A microbial fuel cell is a wastewater treatment apparatus of an energy independent type which, while converting chemical energy of organic matter that is contained in wastewater (domestic wastewater or wastewater from plants) into electrical energy, subjects that organic matter to an oxidative degradation treatment.
A microbial fuel cell includes a negative electrode that carries microorganisms thereon and a positive electrode which is allowed to be in contact with an oxidizing substance, where an electrolytic solution containing organic matter is supplied to the negative electrode, and water containing oxygen is supplied to the positive electrode. The negative electrode and the positive electrode are connected with each other via an external circuit, thereby forming a closed circuit. At the negative electrode, hydrogen ions (H+) and electrons (e−) are generated from the electrolytic solution through microbial catalysis, and these hydrogen ions move to the positive electrode, while these electrons move to the positive electrode via the external circuit. At the positive electrode, the hydrogen ions and the electrons having moved from the negative electrode bind with oxygen (O2), thus being consumed to become water (H2O). During this course, the electrical energy that flows in the closed circuit is collected.
A microbial fuel cell directly produces electrical energy from an organic substrate or the like by virtue of catalysis (metabolic reaction, biochemical conversion) of the microorganisms. Therefore, an improved recovery efficiency is expectable over conventional energy recovery systems which employ a step of converting organic matter into a biogas or the like. Moreover, it is usable not only for power generation, but also as equipment associated with wastewater treatment, organic waste treatment, or organic waste treatment, etc.
In recent years, microbial fuel cells which utilize a gas diffusion electrode as a positive electrode have attracted attention (e.g. Patent Document 1). A gas diffusion electrode is made of a material which may be porous or in the form of a woven fabric, for example, and thus is porous. This structure allows oxygen in a gas phase (e.g., the atmospheric air) to be supplied to the positive electrode. That is, the hydrogen ions and electrons from the negative electrode can be allowed to react with oxygen in a gas phase at the positive electrode.
When a gas diffusion electrode allows oxygen in a gas phase to be supplied to the positive electrode, the following advantages are obtained as compared to the case of supplying dissolved oxygen in water to the positive electrode, for example.
Supplying dissolved oxygen to the positive electrode has the problems of oxidation of the organic matter that is contained in the liquid to be treated, e.g., wastewater, and bottlenecking of power generation by the diffusion velocity of dissolved oxygen. On the other hand, oxygen in a gas phase has a much larger diffusion velocity than the diffusion velocity of dissolved oxygen, thereby being able to efficiently achieve oxidation of the organic matter as well as power generation. Thus, improvements in the output power of the fuel cell can be expected.